test(chain): make `TestLocalChain` generic and add `prev_blockhash` test

test(chain): make `TestLocalChain` generic and add `prev_blockhash` test

Make `TestLocalChain` and `ExpectedResult` generic over checkpoint data
type `D`, allowing the same test infrastructure to work with both
`BlockHash` and `TestBlock` types.

Add `merge_chains_with_prev_blockhash` test to verify that `prev_blockhash`
correctly invalidates conflicting blocks and connects disjoint chains.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>